The present invention relates to a fuel assembly, especially to the fuel assembly preferable for use in boiling water reactors.
In the boiling water reactors, cooling water having moderating function exists in two regions, namely, two phase flow region in the channel box and saturated water region outside of the channel box. In limited space in the reactor core, the optimum value for area ratio of the two regions is different depending on evaluating subjects.
In order to improve fuel economy of the boiling water reactor by achieving high burn up of fuel assemblies, life extension of the fuel assemblies, and uranium saving, there are many methods such as a method of arranging unboiled region, namely, a plurality of water rods, in the fuel assembly, a method of increasing uranium inventory in the fuel assembly, and a method of increasing composing lattice number of fuel rods etc. One of the example of the prior art is the fuel assembly disclosed in JP-A-62-217186 (1987). The fuel assembly has a fuel arrangement of 9 lines by 9 rows, and two water rods having large diameter arranged adjacently in diagonal direction of the fuel rod arrangement. Using the fuel assembly, fuel economy can be improved by increasing of composing lattice number and unboiled region and flattening of power distribution without decreasing the uranium inventory.
And, making void reactivity coefficient close to zero as possible is required for the boiling water reactor in order to improve safety and operability of the reactors.
Void in the boiling water reactor changes depending on power level and coolant flow rate, and causes change of coolant density during reactor operation.
Void reactivity coefficient of the boiling water reactor has a large negative value. The negative void reactivity coefficient has an effect to suppress power increase when reactivity is added. Therefore, if abnormal phenomena which will cause pressure increase in the reactor is generated and such control operation to suppress the pressure increase as control rods insertion is not performed, void in the reactor is decreased, and subsequently rapid power increase is caused by addition of positive reactivity. Consequently, there is a possibility to cause failure of fuel rods by overheating.
Accordingly, lessening of the void reactivity coefficient can moderate the above described abnormal phenomena, and is able to improve safety and operability of the reactor.
As to methods for the lessening of the void reactivity coefficient, a method of increasing water to fuel volume ratio in case of using enriched uranium and plutonium fuel, a method of decreasing water to fuel volume ratio in case of using plutonium fuel, and a method of decreasing void generating area in the case of using enriched uranium and plutonium fuel can be considered.
An example of the methods of increasing water to fuel volume ratio is disclosed in JP-A-63-25592 (1988). The cited reference describes the method in which the cross sectional area of the fuel assembly is increased to almost twice, total cross sectional area of the water rods is increased, and moderator (water) and fuel (uranium) in the fuel assembly are homogenized so as to distribute almost equal quantity of unboiling water to each of fuel rods in order to improve neutron moderating effect and to accelerate effective burning of the uranium by increasing of cross sectional area of the water rods without decreasing the uranium inventory per a fuel assembly.
An example of the methods of decreasing water to fuel volume ratio is disclosed in JP-A-62-259086 (1987). The cited reference describes the method to make the number of M closest integer of 0.87 N when assign N to the number of fuel rods arranged line and M to the number of fuel rods arranged row (both N and m are integers) in a high conversion nuclear reactor loaded with a plurality of fuel assemblies having fuel rods arrangement of triangle lattices. The high conversion nuclear reactor is aimed at positive conversion of uranium-238 to fissile plutonium-239 by decreasing moderating effect on neutron with small water to fuel volume ratio.
In order to decrease the void reactivity coefficient as above described, it is necessary to make remarkable design change to the boiling water reactor such as making the fuel assembly large, or to make the reactor high conversion type etc.